Semiconductor device and method for fabricating the same

Abstract

The on-state characteristics of a transistor are improved and thus, a semiconductor device capable of high-speed response and high-speed operation is provided. A highly reliable semiconductor device showing stable electric characteristics is made. The semiconductor device includes a transistor including a first oxide layer; an oxide semiconductor layer over the first oxide layer; a source electrode layer and a drain electrode layer in contact with the oxide semiconductor layer; a second oxide layer over the oxide semiconductor layer; a gate insulating layer over the second oxide layer; and a gate electrode layer over the gate insulating layer. An end portion of the second oxide layer and an end portion of the gate insulating layer overlap with the source electrode layer and the drain electrode layer.

What is claimed is: 
     
       1. A method for fabricating a semiconductor device, comprising the steps of:
 stacking a first oxide layer and an oxide semiconductor layer; 
 forming a source electrode layer and a drain electrode layer over the first oxide layer and the oxide semiconductor layer; 
 stacking an oxide film and a gate insulating film over the source electrode layer and the drain electrode layer; 
 forming a gate electrode layer over the oxide film and the gate insulating film; 
 etching the oxide film and the gate insulating film to have an island shape using the gate electrode layer as a mask so that a second oxide layer and a gate insulating layer are formed; and 
 forming an oxide insulating layer over the source electrode layer, the drain electrode layer, the second oxide layer, the gate insulating layer, and the gate electrode layer, 
 wherein each of the first oxide layer and the second oxide layer comprises indium and zinc, and 
 wherein a composition of the first oxide layer and a composition of the oxide semiconductor layer are different from each other. 
 
     
     
       2. The method for fabricating the semiconductor device according to  claim 1 , wherein the oxide semiconductor layer comprises indium and zinc. 
     
     
       3. The method for fabricating the semiconductor device according to  claim 1 ,
 wherein an upper surface of the source electrode layer is in direct contact with the second oxide layer, and 
 wherein an upper surface of the drain electrode layer is in direct contact with the second oxide layer. 
 
     
     
       4. The method for fabricating the semiconductor device according to  claim 1 ,
 wherein the source electrode layer comprises a stepped-shape near the drain electrode layer, and 
 wherein the drain electrode layer comprises a stepped-shape near the source electrode layer. 
 
     
     
       5. A method for fabricating a semiconductor device, comprising the steps of:
 stacking a first oxide layer and an oxide semiconductor layer; 
 forming a source electrode layer and a drain electrode layer over the first oxide layer and the oxide semiconductor layer; 
 stacking an oxide film and a gate insulating film over the source electrode layer and the drain electrode layer; 
 forming a gate electrode layer over the oxide film and the gate insulating film; and 
 etching the oxide film and the gate insulating film to have an island shape using the gate electrode layer as a mask so that a second oxide layer and a gate insulating layer are formed, 
 wherein each of the first oxide layer and the second oxide layer comprises indium and zinc, 
 wherein a side surface of the gate insulating layer and a side surface of the gate electrode layer compose a first flat surface, and 
 wherein a composition of the first oxide layer and a composition of the oxide semiconductor layer are different from each other. 
 
     
     
       6. The method for fabricating a semiconductor device according to  claim 5 ,
 wherein a side surface of the second oxide layer and the side surface of the gate insulating layer compose a second flat surface. 
 
     
     
       7. The method for fabricating the semiconductor device according to  claim 5 ,
 wherein an upper surface of the source electrode layer is in direct contact with the second oxide layer, and 
 wherein an upper surface of the drain electrode layer is in direct contact with the second oxide layer. 
 
     
     
       8. The method for fabricating the semiconductor device according to  claim 5 ,
 wherein the source electrode layer comprises a stepped-shape near the drain electrode layer, and 
 wherein the drain electrode layer comprises a stepped-shape near the source electrode layer. 
 
     
     
       9. The method for fabricating the semiconductor device according to  claim 1 ,
 wherein an electron affinity of the oxide semiconductor layer is higher than an electron affinity of the first oxide layer by 0.2 eV or more, and 
 wherein the electron affinity of the oxide semiconductor layer is higher than an electron affinity of the second oxide layer by 0.2 eV or more. 
 
     
     
       10. The method for fabricating the semiconductor device according to  claim 5 ,
 wherein an electron affinity of the oxide semiconductor layer is higher than an electron affinity of the first oxide layer by 0.2 eV or more, and 
 wherein the electron affinity of the oxide semiconductor layer is higher than an electron affinity of the second oxide layer by 0.2 eV or more. 
 
     
     
       11. The method for fabricating the semiconductor device according to  claim 2 , wherein each of the first oxide layer, the second oxide layer, and the oxide semiconductor layer further comprises gallium. 
     
     
       12. The method for fabricating the semiconductor device according to  claim 5 , wherein the oxide semiconductor layer comprises indium and zinc. 
     
     
       13. The method for fabricating the semiconductor device according to  claim 12 , wherein each of the first oxide layer, the second oxide layer, and the oxide semiconductor layer further comprises gallium.